YANGON, Myanmar — A senior Myanmar official on Sunday denied Chinese accusations that one of his country's warplanes crossed China's border and dropped a bomb that killed four farmers during fighting with Myanmar rebels.

The official, from Myanmar's presidential office, said his government regretted the deaths and suggested it could have been the work of a group seeking to create confusion. He also promised "full cooperation" with Beijing into the incident, which has tested generally good ties that have been strained in recent years by Myanmar's perceived shift toward the U.S.

The farmers' deaths Friday occurred as Myanmar's government stepped up its fight against ethnic Chinese rebels in the country's Kokang region along China's southwestern border.

The official, who spoke to The Associated Press on condition of anonymity because of the sensitivity of the matter, cited radar and GPS records as proof that Myanmar fighter jets did not stray into Chinese territory. He also said Yangon always informs Beijing of its airstrikes in the border region beforehand.

Myanmar state television reported that Myanmar's military and Chinese authorities began an investigation Sunday at the site of the incident.

Beijing strongly protested the bombing and said it had sent fighter jets to warn off further intrusion.

Geng Yansheng, spokesman for China's National Defense Ministry, said Saturday that Myanmar warplanes had intruded "multiple times" and that China would take "decisive measures" should they cross over again.

Fan Changlong, vice chairman of the ruling Chinese Communist Party's Central Military Commission, demanded that Yangon seriously investigate the incident, punish those responsible and apologize to the victims' families, according to a statement on the Defense Ministry's website.

Chinese Premier Li Keqiang called the bombing a "very distressful matter." Speaking at a news conference after the close of China's annual legislature in Beijing, he said China was able to "firmly safeguard stability in the border areas."

The Myanmar official said Yangon would extend "full cooperation" with Beijing and wanted to examine the bomb fragments found in China.

Beijing has disavowed any links with the Kokang rebels in Myanmar, saying it respects Myanmar's sovereignty. Myanmar officials have said former Chinese soldiers have trained the rebels, an allegation the insurgents have denied.

Myanmar officials blame the renewed fighting on a renegade rebel faction led by Phone Kya Shin that attempted to seize Laukkai, the capital of the self-administered Kokang region.